The quantity of saturated phosphatidylcholine (SPC) in amniotic fluid was measured in 50 cases of normal late pregnancy and 21 women in labor, by improved Tsai's method. Normal values of SPC in different gestational weeks were obtained. The results showed that SPC in the amniotic fluid increased with the advancing gestational weeks and more significantly after the onset of labor. This study indicated that SPC determination can predict fetal lung maturity with high specificity, sensitivity and accuracy.